HM740/1 is an 8 inputs and 8 outputs unit.

The 8 inputs has both + and – from the opto couplers lead to the terminal block, meaning that activating an input either is done by adding a voltage alternatively pull one of the connections to either 0 or 24V and then add the corresponding to the other connection

HM740/2 is a 16 inputs unit that can detect polarity reversal.

On this unit each input is fitted with two opto couplers, making it possible to monitor the connection. Voltage detected in one direction equals line ok whereas reversed polarity means alarm and missing voltage means line fault

HM740/3 is a 32 inputs unit.

Where the opto couplers are internally pulled to 24V meaning that inputs must be connected to 0V to active an input